Alkaline and acid pressure oxidation of precious metal-containing materials

ABSTRACT

The present invention is directed to a precious metal recovery process in which an acid sulfidic feed material is subjected to acid pressure oxidation and an alkaline sulfidic feed material is subjected to alkaline pressure oxidation, with the discharge slurries from the pressure oxidation processes being combined to reduce neutralization requirements prior to precious metal recovery.

FIELD

The invention relates generally to hydrometallurgical processing ofprecious metal-containing materials and particularly tohydrometallurgical processing of refractory gold ores, concentrates,tailings, and other materials.

BACKGROUND

Many of the world's remaining gold deposits are considered to berefractory or double refractory in nature. Refractory ores are those inwhich the recoveries of gold by conventional cyanidation are typicallynoneconomic. Low gold recoveries can be caused by naturally occurringpreg-robbing carbonaceous materials, which can pre-empt the absorptionof gold by activated carbon in gold recovery processes. In addition,many gold ores are also sulfide refractory. The gold in sulfiderefractory ores is inaccessible to gold lixiviants because the goldoccurs as; particles finely disseminated within sulfide mineral crystalsor as a solid solution in the sulphide matrix. The cost of sizereduction associated with liberating this gold is often prohibitive,and, in the case of gold occurring as a solid solution, the sizereduction is also generally ineffective. This problem has been overcomeby oxidizing the sulfides contained in the ore, thereby liberating thegold from the sulfide matrix and rendering it amenable to leaching bycyanide or other lixiviants. Methods of oxidation employ bio-oxidation,roasting, atmospheric leaching and pressure oxidation. Pressureoxidation can be performed under alkaline conditions, as in the processdisclosed by U.S. Pat. No. 4,552,589, or acid conditions, as disclosedby U.S. Pat. No. 5,071,477.

In alkaline pressure oxidation, the sulfuric acid produced during theoxidation step reacts with the carbonate or other acid consumerscontained in the autoclave feed according to the following reactions(based on pyrite oxidation):

1 Oxidation: 4FeS₂+15O₂+2H₂O→2Fe₂O₃+8H₂SO₄

2 Neutralization: CaCO₃+H₂SO₄→CaSO₄+H₂O+CO₂↑

The amount of carbonate, or other acid consumers, is in stoichiometricexcess to the amount of acid that is generated by oxidation, andtherefore the pH of the autoclave discharge is near neutral to alkalinedepending upon the amount and type of acid consumers. When a portion ofthe gold occurs within the unreacted carbonates in the oxidized ore, thegold can remain inaccessible to lixiviants in subsequent gold leachingoperations and thereby be unrecovered.

In acid pressure oxidation, the following reactions can occur:

3 Oxidation: 4FeS₂+15O₂+2H₂O→2Fe₂O₃+8H₂SO₄

4 Oxidation: 4FeS₂+6O₂+2H₂O→2Fe₂O₃+6S^(o)+2H₂SO₄

The prevalence of one reaction over the other depends on a number offactors including operating temperature, pressure, molecular oxygenoverpressure, and residence time. No matter which of the oxidationreactions prevails, a substantial, amount of sulfuric acid is producedand must be neutralized. Acid neutralization costs can significantlyincrease plant operating costs.

SUMMARY

These and other needs are addressed by the various embodiments andconfigurations of the present invention. The present invention isdirected generally to the parallel use of acid and alkaline pressureoxidation to treat refractory and double refractory sulfidic ores,concentrates, tailings, and other valuable metal-containing materialsand the combination of the autoclave discharges produced therefrom.

The present invention can potentially address the shortcomings of acidand alkaline autoclave discharges. By combining the alkaline autoclavedischarges with the acid pressure oxidation discharge a portion or allof the residual acid from the acid pressure oxidation is neutralizedresulting in a substantial reduction of lime consumption. At the sametime a portion or all of the unreacted carbonates in the alkalinedischarge reacts with the acid, with the potential of liberatingadditional gold.

In one embodiment, an acid generating precious metal-containingautoclave feed material has more acid generators than consumers, and analkaline or acid consuming precious metal-containing feed material hasmore acid consumers than generators. The acid generating feed materialis subjected to pressure oxidation which produces an acidic dischargeslurry while the alkaline autoclave feed material is subjected toalkaline pressure oxidation which produces an alkaline discharge slurry.The acidic and alkaline discharge slurries are combined to provide acombined discharge slurry having a pH higher than that of the acidicdischarge slurry. The combination of the slurries can reduce lime orother alkali consumption thereby significantly reducing neutralizationcosts.

In another embodiment, an aqueous leach barren solution comprising aleaching agent is passed through a membrane filter to form an aqueousretentate (including most of the leaching agent) and an aqueous permeate(including some of the leaching agent). The aqueous retentate isrecycled to a leaching step. Most of the leaching agent in the aqueouspermeate is destroyed to form an aqueous recycle stream substantiallyfree of leaching agent. The aqueous recycle stream is recycled to a unitoperation upstream of the leaching step. The process can inhibit therecycle of lixiviant or leaching agent, in the recycle water, to otherunit operations, which can cause precious metal leaching and consequentlosses. This enables water recycling from the precious metal leach toupstream unit operations, thereby reducing water consumption, andpermitting leaching agent recycle to leaching, thereby reducing reagentcosts. Membrane filtration beneficially reduces the volume of leachingsolution recycled to precious metal leaching and the concentration ofleaching agent needing to be destroyed (thereby reducing reagent costs).

The embodiments of the present invention can provide a number of otheradvantages depending on the particular configuration. The process cancombine the advantages of alkaline and acid pressure oxidation whileoffsetting their respective disadvantages. While alkaline pressureoxidation effectively treats highly carbonate materials, it cannoteconomically or effectively treat highly sulfidic materials. Incontrast, acid pressure oxidation effectively treats highly sulfidicmaterials but not highly carbonate materials. The selective use of acidpressure oxidation to treat sulfidic materials having relatively lowcarbonate levels and alkaline pressure oxidation to treat carbonatematerials having relatively low sulfide sulfur levels is more effectivethan blending the materials and treating them in the autoclave together.The generation of carbon dioxide within the autoclave from the reactionof generated sulfuric acid with carbonates is to be minimized wheneverpossible, as the generation of CO₂ within the autoclave will causeincreased venting of the autoclave, which in turn causes the loss of O₂and increased O₂ consumption and heating costs. For this reason,pre-acidification is commonly employed for low carbonate sulfidic ores.Pre acidification is generally not cost effective on high carbonatesulfide ores.

These and other advantages will be apparent from the disclosure of theinvention(s) contained herein.

As used herein, “at least one”, “one or more”, and “and/or” areopen-ended expressions that are both conjunctive and disjunctive inoperation. For example, each of the expressions “at least one of A, Band C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “oneor more of A, B, or C” and “A, B, and/or C” means A alone, B alone, Calone, A and B together, A and C together, B and C together, or A, B andC together.

The term “a” or “an” entity refers to one or more of that entity. Assuch, the terms “a” (or “an”), “one or more” and “at least one” can beused interchangeably herein. It is also to be noted that the terms“comprising”, “including”, and “having” can be used interchangeably.

The term “acid consumer” refers to any compound that reacts with an acidto produce a salt or water.

The term “autoclave feed material” refers to all materials fed into theautoclave, For example, in acid POX the autoclave feed material mayinclude additional acid, or may refer to ores which have beenpre-acidified to reduce the level of acid consumers prior to theaddition to the autoclave. In alkaline POX the feed material may includeadditional alkaline sources

The term “acid equivalent” refers to an acid, which does not involve aproton transfer.

The term “acid generator” refers to a compound that will generatesulfuric acid. The most common form of acid generator is sulfide sulfur.A mole of an acid generator is defined as that amount which, under theparticular pressure oxidation conditions, generates one mole of sulfuricacid. The “moles of total acid generators” is the sum of the moles ofall acid generators present.

The term “autoclave” refers to any reactor that effects oxidation undersuper atmospheric conditions. An autoclave can be a single,multi-compartmented reactor or a reactor system including multiple,separate reactor vessels.

The term “carbonaceous material” and “TCM” refers to organiccarbon-containing material. Examples of organic carbonaceous materialsinclude humic acid, hydrocarbons, and activated carbon.

The term “dissociate” or “dissociable” and variations thereof refers tothe process in which ionic compounds (complexes, molecules, or salts)separate or split into smaller molecules, ions, or radicals.

The term “dissolve” and variations thereof refer to is the process bywhich a solid or liquid enters its aqueous phase (solution).

The term “free sulfuric acid” refers to the excess sulfuric acid overthe moles of total acid consumers present in the material.

The term “inorganic carbon” refers to binary compounds of carbon such ascarbon oxides, carbides, carbon disulfides, etc., ternary compounds, andthe metallic carbonates, such as calcium carbonate and sodium carbonate.

The term “mineral” and variations thereof refer to any naturally formedchemical substance having a definite chemical composition andcharacteristic crystal structure.

A “mole of an acid consumer” is defined as that amount which, under theparticular pressure oxidation conditions, reacts with (consumes) onemole of sulfuric acid. The “moles of total acid consumers” is the sum ofthe moles of all acid consumers present. Exemplary classes of acidconsumers include carbonates, oxides and hydroxides of metals. Acidconsumers are commonly compounded with alkali and alkaline earth metals.Specific examples of acid consumers include carbonates, such aslimestone, soda ash, trona, dolomite, and calcite; metal oxides such aslime, zinc oxide, magnesium oxide; hydroxides such as sodium hydroxide,potassium hydroxide, ammonia, ferric hydroxide, laterite, limonite,goethite, gibbsite, and diaspore; and various clays.

The term “precious metal” refers generally to gold, silver, and theplatinum group or platinum metals. The platinum metals refer to a groupof six metals, all members of group VIII of the Periodic Table of theElements: ruthenium, rhodium, palladium, osmium, iridium, and platinum.

The term “solution derived therefrom” refers to a solution having atleast one common component with the source solution from which thesolution is derived, directly or indirectly. For example, a solutionhaving a leaching agent, contaminant, or valuable metal found in thesource solution is deemed to be derived therefrom. Thus, a raffinate orbarren solution is deemed to be a solution derived from a pregnant leachsolution. Likewise, a loaded extractant or electrolyte, which containsthe valuable metal, or strip solution are deemed to be derived, directlyor indirectly, from the pregnant leach solution.

The term “total acid” refers to both the free acid and other acidequivalents. Dissolved ferric sulfate and solid basic ferric sulfate areexamples of acid equivalents.

The preceding is a simplified summary of the invention to provide anunderstanding of some aspects of the invention. This summary is neitheran extensive nor exhaustive overview of the invention and its variousembodiments. It is intended neither to identify key or critical elementsof the invention nor to delineate the scope of the invention but topresent selected concepts of the invention in a simplified form as anintroduction to the more detailed description presented below. As willbe appreciated, other embodiments of the invention are possibleutilizing, alone or in combination, one or more of the features setforth above or described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of thespecification to illustrate several examples of the presentinvention(s). These drawings, together with the description, explain theprinciples of the invention(s). The drawings simply illustrate preferredand alternative examples of how the invention(s) can be made and usedand are not to be construed as limiting the invention(s) to only theillustrated and described examples. Further features and advantages willbecome apparent from the following, more detailed, description of thevarious embodiments of the invention(s), as illustrated by the drawingsreferenced below.

FIGS. 1A-B are block diagrams depicting a processing circuit fortreating precious metal-containing materials according to an embodiment;and

FIGS. 2A-C are block diagrams depicting a process for treating preciousmetal-containing materials according to an embodiment; and

FIG. 3 is a block diagram depicting a process for regenerating aleaching solution according to an embodiment.

DETAILED DESCRIPTION

Referring to FIGS. 1A-B and 2A-C, the various process embodimentsdisclosed herein are directed to the simultaneous treatment of an acidgenerating precious metal- and sulfide-containing material 100 bypressure oxidation producing an acidic discharge slurry 228 and an acidconsuming precious metal- and sulfide containing material 104 bypressure oxidation producing an alkaline discharge slurry 252. Pressureoxidation liberates precious metals from the refractory sulfide mineralmatrix. The materials 100 and 104 are typically in the form of an ore,concentrate, tailings, such as reverse flotation tailings, aconcentrate, such as a flotation concentrate, calcine, recycledindustrial matter, or any other form from which precious metal may berecovered. The materials 100 and 104 typically contain no less thanabout 0.01 and even more typically from about 0.05 to about 10 (troy)ounces of precious metal (typically gold) per ton.

The acid generating precious metal- and sulfide-containing material 100typically contains from about 1 weight percent to about 35, even moretypically from about 2 to about 15, weight percent sulfide sulfur Thematerial, as it enters the autoclave 120, contains an equivalentcarbonate level, in the material, of preferably between about 0.1 and 1%by weight In some applications, the material 100, may be pre-acidulatedto achieve the desired carbonate level. For ores such as those ofsulfide-containing material 104, pre-acidulation to this extent ofcarbonate content is not economical.

The acid consuming or alkaline precious metal and sulfide containingmaterial 104, on the other hand, typically contains, or is adjusted tocontain, an equivalent carbonate level, in the material, of greater thanabout 0.7% by weight as the material enters the autoclave 136. Commonly,the material 104 contains from about 1 to about 45 percent by weighttotal inorganic carbon compounds (based on CO₃). Most, and typically atleast about 90%, of the inorganic carbon present in the ores andconcentrates is in the form of inorganic metal carbonates, such aslimestone, dolomite, calcite, and magnesite. Carbonate ores andconcentrates can contain sulfide sulfur primarily in the form ofsulfides of iron, arsenic, and other metals. Commonly, carbonate oresand concentrates contain up to about 15 weight percent total sulfidesulfur and typically from about 0.1 to about 7 weight percent totalsulfide sulfur.

The precious metal- and sulfide-containing materials 100 and 104 can beobtained in many ways. In one application, the materials 100 and 104 areexcavated from different parts or zones of a stratified deposit. Inother applications, the materials 100 and 104 are produced by floating acommon feed material. The low carbonate sulfide-containing material 100,for example, can be a concentrate or tailings fraction. In anotherapplication, the materials 100 and 104 are excavated from differentdeposits.

In steps 200 and 204, respectively, the sulfide-containing low carbonateand high carbonate-containing materials 100 and 104, respectively, arecomminuted and slurried by the corresponding acid and alkalinecomminution circuits 108 and 112 to produce slurried comminuted preciousmetal- and sulfide-containing and carbonate-containing materials 208 and212, respectively. The acid and alkaline comminution circuits 108 and112 can be any open or closed crushing and/or grinding circuits andthickeners known to those of ordinary skill in the art. Typicalcomminution devices include autogeneous, semi-autogeneous, ornon-autogeneous crushers, ball mills, rod mills, and the like.Thickening of the slurried comminuted materials is typically performedusing polymeric flocculants. Although the size fraction depends on themineralogies of the materials 100 and 104 and the liberation of theprecious metal, the comminuted materials 208 and 212 preferably have aP₈₀ size of about 20 microns or higher, even more preferably of about 35microns or higher, and even more preferably ranging from about 40 toabout 150 microns. The slurried materials 100 and 104 typically have apulp density of from about 10 to about 60, even more typically of fromabout 30 to about 55, and even more typically from about 50 to about 55wt. % solids.

Acid Pressure Oxidation

Before pressure oxidation, the optionally pre-acidulated material 220may be preheated, particularly when processing low sulfur feeds. Steamfrom the acid autoclave 120 and/or from the flash cooler 128 iscontacted with the acidulated material, such as by splash condensers, toheat the material to a preferred temperature ranging from about 30 toabout 115° C.

In step 224, the acidulated material 220 is subjected to pressureoxidation to oxidize preferably most, even more preferably about 75% ormore, and even more preferably about 90% or more of the sulfide sulfurand form an acid autoclave discharge slurry 228. The oxidation productdepends on the temperature regime. When the pressure oxidationtemperature is in the range of from about 100 to about 170° C., asignificant portion of the sulfide sulfur is converted into elementalsulfur. When the pressure oxidation temperature is above about 170 toabout 240° C., sulfide sulfur is irreversibly oxidized to sulfatesulfur. In either case sulfuric acid is produced.

Most of the acid consumers in the (optionally preacidulated) material220 will react with the generated sulfuric acid to yield a preferred pHof less than pH 7 and even more preferably ranging from about pH 0 toabout pH 6.5. For example, carbonates present in the (optionallypreacidulated) material 220 will react with the generated sulfuric acid,producing gypsum, water and carbon dioxide. To minimize the effect ofcarbon dioxide production, it is common practice to bleed off a portionof the gas in the vapor space to prevent carbon dioxide build-up and torecycle the large tail gas stream from pressure oxidation through anacid gas scrubber for particulate and sulfur dioxide removal.

The preferred reactor vessel for pressure oxidation is a sealed,multiple-compartment autoclave. The autoclave typically has at least sixcompartments to minimize short circuiting of the autoclave feed material216 to the acid autoclave discharge slurry 228 as can occur inautoclaves with fewer compartments.

During pressure oxidation, sufficient molecular oxygen is introducedinto the acid autoclave 120 to preferably maintain a molecular oxygenpartial pressure ranging from about 20 to about 300 psig. The totalpressure in the acid autoclave is preferably in the range of from about250 to about 750 psig.

The duration of pressure oxidation depends upon a number of factors,including the mineral characteristics of the autoclave material 220 andthe pressure oxidation temperature and pressure, particle size, and freeacid levels. Preferably, the duration of pressure oxidation ranges fromabout 0.5 to about 3 hours.

The acid autoclave discharge slurry 228 contains a liquid phaseincluding a variety of species, including dissolved (iron) sulfatesulfur and commonly from about 2 to about 150 g/L, more commonly lessthan about 25 g/L, even more commonly less than about 10 g/L, and evenmore commonly between about 5 and 25 g/L free sulfuric acid and a solidphase, including preferably most, more preferably about 95% or higher,and even more preferably about 99% or higher of the precious metal.

The acid autoclave discharge slurry 228, in step 232, is flashed in anopen vessel or flashed cooler 124 to release pressure and evaporativelycool the slurry 228 through release of steam to form a flashed slurry.The flashed slurry typically has a pulp density of 30% by weight orhigher.

In optional hot cure step 236, the acid autoclave discharge slurry 228slurry is maintained, in one or more hot cure vessel(s) 128, at apreferred temperature of about 60° C. or higher, for a time sufficientfor most of the solid-phase basic ferric sulfate to react with freesulfuric acid in the liquid phase to form dissolved ferric sulfate. Thedetails of the hot cure step are disclosed in U.S. patent applicationSer. Nos. 11/761,103, filed Jun. 11, 2007, and 11/249,120, filed Oct.11, 2005, (now U.S. Pat. No. 7,604,703), each of which is incorporatedherein by this reference. The hot cure step 236 can reduce,significantly, lime consumption in later neutralization.

Alkaline Pressure Oxidation

The comminuted precious metal- and carbonate-containing material 212, inparallel, is conditioned in step 240, in one or more conditioningvessels 132, to form a conditioned material having a desired slurry pulpdensity (which is in the range set forth above) and containing anconditioning agent to inhibit, during pressure oxidation, the coating ofthe material 212 with insoluble metal compounds. While any suitableconditioning agent may be employed, preferred conditioning agentsinclude trona (Na₃(CO₃)(HCO₃)·2(H₂O)), NaCO₃, NaOH, and mixturesthereof. For example, trona, when added in sufficient quantities, cansubstantially inhibit the formation of gypsum. Preferably, the amount ofconditioning agent added ranges from about 5 to about 40 lb/ston andeven more preferably from about 10 to about 30 lb/short ton (ston.)

The conditioned material 244 is subjected to alkaline pressureoxidation, in an alkaline autoclave 136 and in step 248 to form analkaline autoclave discharge slurry 252. Alkaline pressure oxidationoxidizes sulfide sulfur to form sulfuric acid, which is neutralized bythe acid consumers, such as inorganic metal carbonates (including thetrona), in the conditioned material 244. Alkaline pressure oxidation(though operated under neutral or slightly alkaline pH), uses similarconditions of temperature (in the above-quoted temperature range), totalpressure (in the above-quoted pressure range), molecular oxygen partialpressure (in the above-quoted molecular oxygen partial pressure range),autoclave residence time (in the above-quoted residence time range), asacid pressure oxidation. A primary difference is that typically most,and even more typically about 75% or more, and even more typically about95% or more of the sulfuric acid generated by sulfide oxidation in theconditioned material 244, is rapidly neutralized by acid consumers inthe material 244. In addition, unlike acid pressure oxidation, thedegree of gold recovery is not as closely tied to the degree of sulfideoxidation, for or this reason the economic limit of percent sulfideoxidation of the alkaline material can be as low as 50%. The pH of thealkaline autoclave discharge slurry 252 is preferably about pH 6 orhigher, even more preferably ranges from about pH 7 to about pH 9, andeven more preferably ranges from about pH 7 to about pH 8. The pH of thedischarge from the alkaline autoclave may be below 7, however if theacid base reaction is permitted to reach steady state, the pH will bebetween about pH 6 to about pH 8.

The alkaline autoclave discharge slurry 228 contains a liquid phasecommonly having no more than about 1 g/L free sulfuric acid and acidequivalents. and a solid phase, preferably including preferably most,more preferably about 95% or higher, and even more preferably about 99%or higher of the precious metal.

The alkaline autoclave discharge slurry 252, output by the alkalineautoclave 136, is introduced into a flash cooler 140 and subjected toflash cooling in step 256.

Combination of Acid and Alkaline Autoclave Discharge Slurries

The acid and alkaline autoclave discharge slurries 228 and 252, afterflash cooling 232 and 256 and optionally hot curing step 236, ispreferably further cooled in indirect heat exchanges ahead ofneutralization and precious metal recovery. Depending on the temperatureafter flash cooling, the acid autoclave discharge slurry may besubjected to indirect heat exchange cooling before the hot curing step236. Although intermediate washing and/or liquid-solid separation may beemployed, direct neutralization after cooling is preferred. Thetemperature of the hot acid autoclave discharge slurry is preferablyreduced to between about 90° F. and about 140° F., more preferablybetween about 110° F. and about 130° F., and even more preferably belowabout 120° F.

In step 260, the partially cooled acid and alkaline autoclave dischargeslurries 228 and 252 are combined, in one or more combination vessels144 to form a combined discharge slurry 264, in which acid contained inthe acid discharge 228 reacts with the carbonates and other acidconsumers in the alkaline discharge. It is not necessary that the acidand alkaline discharge slurries be of equivalent volume. The pH of thecombined slurry will be greater than that of the acid discharge, andlower than that of the alkaline discharge. As will be appreciated, thefinal pH of the combined discharge slurry 264 can be acidic or alkalineand varies depending on the relative volumes and acid consumer on theone hand and free acid and acid equivalent on the other hand contents ofthe acid and alkaline autoclave discharge slurries 228 and 252. In oneapplication, the combined discharge slurry has less free sulfuric acidand/or acid equivalents than the acid discharge slurry 228 and/or morefree sulfuric acid and/or acid equivalents than the alkaline dischargeslurry 252. In one application, the combined discharge slurry has moreacid consumers than the acid discharge slurry 228 and/or less acidconsumers than the alkaline discharge slurry 252. The slurries arecontained in a stirred neutralization vessel for 30 to 120 minutes.

Once the reaction between the acid and alkaline discharge is complete,the pH of the combined slurry is adjusted, if required, in step 270 andin one or more neutralization vessels 170 to a level suitable for thelixiviant employed, which is typically a pH of about pH 8 or higher.

The neutralization vessel(s) 170 can be in a single stirred stage or inmultiple stirred stages, with two to six stages being common. Under theagitated conditions, decomposition occurs more rapidly because theresidue is dispersed and suspended for more efficient contact with theacid consumer. The acid consumer is added while agitating the removedsolid phase. The amount of lime required will depend upon the terminalpH of the combined slurry, and the target pH for gold recovery.

In step 276 and precious metal recovery circuit 178, the precious metalsare dissolved by leaching the solid phase of the neutralized dischargeslurry 272. Leaching is typically performed without additionalliquid/solid separation or pulp density adjustment operations beingperformed on the neutralized discharge slurry 272. The leaching agent orlixiviant is typically cyanide, ammonium, sodium or calcium thiosulfateand less typically halides (iodide, bromide, chloride), and thiourea. Inone configuration, the leaching step 276 is performed at atmosphericpressure and under alkaline conditions (at or above a pH of about pH 7)to produce a pregnant leach solution containing (at least) most of theprecious metal content of the slurry 272. The precious metal leachingstep 276 may be performed by any suitable technique including usingcyanide leaching and Carbon-in-Pulp or CIP techniques, Carbon-In-Leachor CIL techniques, cementation techniques, Resin-in-Pulp or RIPtechniques, Resin-In-Leach or RIL techniques, or by circulating apregnant leach solution and/or slurry through one or more precious metalsorbent columns. In the CIL, CIP, RIP, RIL, and other sorbent-basedtechniques, a sorbent, such as activated carbon or an ion exchangeresin, sorbs the precious metal dissolved in the lixiviant. The sorbedprecious metal is stripped from the sorbent by an suitable eluant toform a barren sorbent for recycle to the leaching step 276 with and/orwithout regeneration, and a pregnant eluate containing most of theprecious metal sorbed on the sorbent.

In the precious metal recovery step 280, the precious metal is recoveredfrom the pregnant leach solution (or pregnant eluate) by suitabletechniques, such as electrowinning or cementation followed by smelting,to form the precious metal product 284. When required, the barren solidresidue or tailings 288 from the leaching step 276 is subjected toleaching agent detoxification or destruction and discarded at tailingsdisposal or impoundment area 182.

An embodiment of the precious metal recovery circuit 178 and process 280will be discussed with reference to FIG. 3. In the depicted process, abarren leach solution, containing the lixiviant, is subjected tomembrane filtration to recycle a retentate to the lixiviant to aprecious metal-containing residue leaching step and a permeate to alixiviant destruction step, with the lixiviant destroyed recycle waterbeing used in other upstream process steps, such as comminution.

With reference to FIG. 3, the precious metal-containing residue 300 issubjected to precious metal-containing residue leaching in step 304 (asdiscussed above) to dissolve most of the precious metals from theresidue and form a pregnant leach solution 308. In step 312, most of theprecious metal is recovered, by any of the techniques noted above, fromthe pregnant leach solution 308 to form the precious metal product 284and the barren leach solution 316 substantially free of dissolvedprecious metals. The barren leach solution 316 can be isolated from theresidue by known liquid/solid separation techniques, such as thickening.

The barren leach solution 316, after optional gypsum removal step 370,is directed to membrane filtration step 320 to form a retentate 324containing most of the lixiviant, or leaching agent, in the barren leachsolution 316, and a permeate 328 containing some leaching agent and mostof the water in the barren leach solution 316. Gypsum removal step 370is performed when the dissolved gypsum exceeds the gypsum solubilitylimit that can be handled by the anti-scalant. Anti-sealant reagentcosts can be decreased by decreased gypsum load following step 370. Aswill be appreciated, the anti-sealant is added following optional step370 and before the membrane filtration step 320. The retentate iscommonly less than 50% of the volume of the barren leach solution 316while the permeate is commonly more than 50% of the barren leachsolution volume. Preferably, only a single membrane filtration stage isemployed to enable reduced osmotic pressure and avoid gypsum scalingfrom highly concentrated salts in subsequent filtration stages. Thebarren leach solution 316 may be softened with a water softener, such assoda ash, prior to filtration. The membrane filter may be any filterable to remove most of the lixiviant in the retentate. Preferably, fromabout 75 to about 95 and even more preferably from about 80 to about92.5% of the leaching agent is separated into the retentate, leavingfrom about 5 to about 25% of the leaching agent in the permeate.Exemplary membrane filters include leaky reverse osmosis filters,nanofilters, ultrafilters, and microfilters.

The retentate 324 is recycled to the precious metal-containing leachingstep 304. Prior to recycling, sodium carbonate is added to the retentate324 stream, and precipitated gypsum is removed by clarification in thegypsum removal step 350, thus lowering the sulfate level circulatingback to the leaching step 304 Gypsum precipitation is believed to resultfrom decomposition of the anti-scalant added upstream of the membranefiltration step 320. Precipitated gypsum is recycled to step 350 topromote calcium removal. The recycled gypsum acts as a “seed” ornucleation site for further gypsum precipitation. When the lixiviantoxidatively decomposes, such as thiosulfate oxidatively decomposes toform polythionates, the retentate 324 may be regenerated in lixiviantregeneration step 360 before reuse in the leaching step 304. As will beappreciated, thiosulfate can be regenerated from polythionates bysulfite, sulfide, or polysulfide addition, and by thermal techniques.

The permeate 328 is oxidized in lixiviant destruction step 332 todestroy most, and even more commonly 95% or more of the residualleaching agent and form recycle water 336. The destruction products aresubstantially unable to leach precious metal(s). The particular processused to effect destruction depends on the leaching agent. Exemplaryprocesses include volatilization (e.g., by pH or temperature changes,solution aeration or agitation, and combinations thereof), chemical orbiological oxidation, adsorption, iron complexation, and precipitation.

In one application, an oxidation process is used in step 332 to destroythe lixiviant. The particular oxidation process depends on theparticular leaching agent employed. Known chemical oxidants includehydrogen peroxide, sulfur dioxide/air, alkaline chlorine-hypochlorite,Caros acid, and ozone. By way of example, thiosulfate destruction can bedone by reagents, such as hydrogen peroxide, and the SO₂/air.Thiosulfate, Caros acid, and residual sulfur species are oxidized tosulfates. As will be appreciated, excess oxidants can degrade theleaching agent in precious metal leaching. Accordingly, most of theresidual oxidants are preferably consumed by super-stoichiometricaddition of reductants. In one configuration, excess hydrogen peroxideis reacted with Caro's acid to form sulfates.

The recycle water 336, which is substantially free of the leaching agentand residual oxidants, is recycled to other unit operations.

EXPERIMENTAL

The following examples are provided to illustrate certain embodiments ofthe invention and are not to be construed as limitations on theinvention, as set forth in the appended claims. All parts andpercentages are by weight unless otherwise specified.

Example 1 Acid Pressure Oxidation Followed by Gold Recovery by Resin inLeach

The following double refractory gold ore was ground to P₈₀ 75 micronsand pre-acidulated to remove excess carbonate prior to treatment bypressure oxidation. The assay for the feed and pre-acidulated feedmaterial were:

Acidualted Element Unit Feed Assay Assay Gold ppm 2.59 Total Sulfur %1.36 Sulfide % 0.90 0.86 Carbonate % 5.28 0.05 TCM % 0.20

Batch pressure oxidation was completed on 625 grams of the acidulatedsample at a feed pulp density of 40% for a period of one hour at 200°C., with an oxygen over pressure of 80 psi. The pH of the autoclavedischarge was 1.3. The sulfide level in the pressure oxidation dischargesolids was measured at 0.07%, and therefore the percent sulfideoxidation was 92.7%. The pH of the discharge was adjusted and maintainedat pH 8 using 39.5 kg Ca(OH)₂/tonne of ore. The gold was then recoveredusing strong base macroporous resin in leach, in which the goldlixiviant, calcium thiosulfate at a concentration 0.1M S₂0₃ wasemployed. The percent gold recovered was 70.8%.

Example 2 Alkaline Pressure Oxidation Followed by Gold Recovery by Resinin Leach

The following double refractory gold ore was ground to P₈₀ 75 micronsand unlike the sample in example 1, it was not pre-acidulated. The assayfor the feed was:

Element Unit Assay Gold ppm 10.7 Total Sulfur % 2.06 Sulfide % 1.49Carbonate % 13.1 TCM % 0.15

Batch pressure oxidation was completed on 625 grams of sample at a feedpulp density of 40% for a period of one hour at 225° C., with an oxygenover pressure of 80 psi. 20 lb/ton of trona was added to the feedmaterial prior to pressure oxidation. The pH of the autoclave dischargewas 8.0. The sulfide content of the discharge was 0.61, and thereforethe percent sulfide oxidation was 60.5%. The pH of the discharge wasadjusted and maintained at pH 8 using 0.6 kg Ca(OH)₂/tonne of ore. Thegold was then recovered using strong base macroporous resin in leach, inwhich the gold lixiviant, calcium thiosulfate at a concentration 0.1MS₂O₃ was employed. The percent gold recovered was 80.0%.

Example 3 Acid and Alkaline Pressure Oxidation Combined DischargeFollowed by Gold Recovery by Resin in Leach

The pressure oxidation steps were repeated using the same feed materialsand conditions as outlined in Examples 1 and 2. The separate dischargesfrom the autoclave were analyzed and details are shown in Table 3.

Acid Pressure Alkaline Pressure Element Unit Oxidation DischargeOxidation Discharge pH — 1.3 8.0 Carbonate % N.A N.A. Sulfide % N.A N.A Gold ppm  N.A. N.A.

The two discharge slurries were then combined 50/50 and mixed for aperiod of 30 minutes with a resulting pH of 6.77. The pH of the combineddischarge is higher than that of the acid pressure oxidation dischargeand lower than of the alkaline pressure oxidation discharge. 3.5 kgCa(OH)₂/tonne of ore was added to adjust the pH to 8 prior to goldrecovery by resin in leach. The lime consumption for the combineddischarge was 36 kg/t less than that required to neutralize Example 1acid discharge, and only 2.9 kg/t more than that required to adjustExample 2 alkaline discharge, indicating that the combination of the twodischarge streams results in significant, reduction of lime consumption.The gold was then recovered using resin in leach, as outlined above. Thepercent gold recovered was 74.8, which is the median value of the goldrecoveries achieved in examples 1 (70.8%) and 2 (80.0%).

Example 4 Combined Feed to Alkaline Pressure Oxidation CombinedDischarge Followed by Gold Recovery by Resin in Leach

Equal amounts of the acid feed material used in examples 1 and thealkaline feed used in example 2 were blended to produce an autoclavefeed material containing:

Element Unit Assay Gold ppm 6.73 Total Sulfur % Sulfide % 1.09 Carbonate% 8.84 TCMBecause of the high carbonate level in the combined feed alkalinepressure oxidation was performed. Batch pressure oxidation was completedon 625 grams of sample at a feed pulp density of 40% for a period of onehour at 225° C., with oxygen over pressure of 80 psi. 20 lb/ton of tronawas added to the feed material prior to pressure oxidation. The pH ofthe autoclave discharge was 7.6. The sulfide content of the dischargewas 0.48%, and therefore the percent sulfide oxidation was 58.6—lowerthan the sulfide oxidation observed in examples 1 (92.7%) and 2 (60.5%).The pH of the discharge was adjusted to 8 using 1.5 kg Ca(OH)₂/tonne ofore. This is slightly lower than the lime consumption in example 3. Thegold was then recovered using resin in leach, and the percent goldrecovered was 74.5%. This is the same as the gold recovery in example 3.

A number of variations and modifications of the invention can be used.It would be possible to provide for some features of the inventionwithout providing others.

The present invention, in various embodiments, configurations, oraspects, includes components, methods, processes, systems and/orapparatus substantially as depicted and described herein, includingvarious embodiments, configurations, aspects, subcombinations, andsubsets thereof. Those of skill in the art will understand how to makeand use the present invention after understanding the presentdisclosure. The present invention, in various embodiments,configurations, and aspects, includes providing devices and processes inthe absence of items not depicted and/or described herein or in variousembodiments, configurations, or aspects hereof, including in theabsence, of such items as may have been used in previous devices orprocesses, e.g., for improving performance, achieving ease and\orreducing cost of implementation.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of theinvention are grouped together in one or more embodiments,configurations, or aspects for the purpose of streamlining thedisclosure. The features of the embodiments, configurations, or aspectsof the invention may be combined in alternate embodiments,configurations, or aspects other than those discussed above. This methodof disclosure is not to be interpreted as reflecting an intention thatthe claimed invention requires more features than are expressly recitedin each claim. Rather, as the following claims reflect, inventiveaspects lie in less than all features of a single foregoing disclosedembodiment, configuration, or aspect. Thus, the following claims arehereby incorporated into this Detailed Description, with each claimstanding on its own as a separate preferred embodiment of the invention.

Moreover, though the description of the invention has includeddescription of one or more embodiments, configurations, or aspects andcertain variations and modifications, other variations, combinations,and modifications are within the scope of the invention, e.g., as may bewithin the skill and knowledge of those in the art, after understandingthe present disclosure. It is intended to obtain rights which includealternative embodiments, configurations, or aspects to the extentpermitted, including alternate, interchangeable and/or equivalentstructures, functions, ranges or steps to those claimed, whether or notsuch alternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

What is claimed is:
 1. A process, comprising: (a) pressure oxidizing, ata pH of less than pH 6.5, an acid generating feed material comprising avaluable metal and sulfide sulfur to form an acid discharge slurrycomprising sulfate sulfur and having a first pH of less than pH 6.5; (b)pressure oxidizing, at a pH of at least pH 6.5, an acid consuming feedmaterial comprising the valuable metal and sulfide sulfur to form analkaline discharge slurry comprising sulfate sulfur and having a secondpH of at least pH 6.5; and (c) combining the acid and alkaline dischargeslurries to provide a combined discharge slurry having a third pHgreater than the first pH and less than the second pH.
 2. The process ofclaim 1, wherein the valuable metal is a precious metal, wherein theacid generating feed material comprises more total moles of acidgenerators than acid consumers, and wherein the acid consuming feedmaterial comprises more total moles of acid consumers than acidgenerators.
 3. The process of claim 2, wherein an equivalent carbonatelevel in the acid generating feed material ranges from about 0.1 toabout 1% by weight and wherein an equivalent carbonate level in the acidconsuming feed material is at least about 0.7% by weight.
 4. The processof claim 1, wherein the acid discharge slurry comprises at least about 2g/L free sulfuric acid while the alkaline discharge slurry comprises nomore than about 1 g/L free sulfuric acid and acid equivalents.
 5. Theprocess of claim 1, wherein at least most of the sulfide sulfur in theacid generating, and acid consuming feed materials is oxidized in steps(a) and (b) and wherein at least most of the oxidized sulfide sulfur inthe acid and alkaline discharge slurries is in the form of sulfatesulfur.
 6. The process of claim 1, wherein the acid discharge slurrycomprises from about 2 to about 150 g/L free sulfuric acid and acidequivalents, wherein the alkaline discharge slurry comprises no morethan about 1 g/L free sulfuric acid and acid equivalents, wherein thecombined discharge slurry has an acidic pH and contains less freesulfuric acid and acid equivalents than the acid discharge slurry andmore free sulfuric acid and acid equivalents than the alkaline dischargeslurry, and wherein the valuable metal is gold and further comprising:(d) after step (c), further increasing a pH of the combined dischargeslurry to form a combined feed slurry having a fourth pH, the fourth pHbeing equal to or higher than the third pH; and (e) recovering gold froma solid phase of the combined feed slurry.
 7. The process of claim 1,wherein the acid discharge slurry comprises from about 2 to about 150g/L free sulfuric acid and acid equivalents, wherein the alkalinedischarge slurry comprises no more than about 1 g/L free sulfuric acidand acid equivalents, wherein the combined discharge slurry has analkaline pH and contains less acid consumers than the alkaline dischargeslurry and more acid consumers than the acid discharge slurry, andwherein the valuable metal is gold and further comprising: (d) afterstep (c), further increasing a pH of the combined discharge slurry toform a combined feed slurry having a fourth pH, the fourth pH beinghigher than the first, second, and third pH's; and (e) recovering goldfrom a solid phase of the combined feed slurry.
 8. A process,comprising: (a) pressure oxidizing, at a pH of less than pH 6.5 and inan acid autoclave, an acid Generating and, precious metal-containingfeed material comprising an acid generator and acid consumer to form anacid generating and, discharge slurry comprising generated acid andhaving a first pH of less than pH 6.5; (b) pressure oxidizing, at a pHof at least pH 6.5 and in an autoclave, an acid consuming and preciousmetal-containing feed material comprising the acid generator and acidconsumer to form an alkaline discharge slurry comprising sulfate sulfurand having a second pH of at least pH 6.5; (c) combining the acid andalkaline discharge slurries to provide a combined discharge slurryhaving a third pH greater than the first pH and less than the second pH;and (d) thereafter recovering the precious metal from a solid phase ofthe combined discharge slurry.
 9. The process of claim 8, wherein theacid generating and precious metal-containing feed material comprisesmore total moles of acid generators than acid consumers and wherein theacid consuming and precious metal-containing feed material comprisesmore total moles of acid consumers than acid generators.
 10. The processof claim 9, wherein an equivalent carbonate level in the acid generatingand precious metal-containing feed material ranges from about 0.1 toabout 1% by weight and wherein an equivalent carbonate level in the acidconsuming and precious metal-containing feed material is at least about0.7% by weight.
 11. The process of claim 8, wherein the acid generatoris primarily sulfide sulfur and wherein the acid discharge slurrycomprises at least about 2 g/L but less than about 25 g/L, free sulfuricacid and acid equivalents while the alkaline discharge slurry comprisesno more than about 1 g/L free sulfuric acid and acid equivalents. 12.The process of claim 11, wherein at least most of the sulfide sulfur inthe acid generating and acid consuming feed materials is oxidized insteps (a) and (b) and wherein at least most of the oxidized sulfidesulfur in the acid and alkaline discharge slurries is in the form ofsulfate sulfur.
 13. The process of claim 8, wherein the acid dischargeslurry comprises from about 2 to about 150 g/L free sulfuric acid andacid equivalents, wherein the alkaline discharge slurry comprises nomore than about 1 g/L free sulfuric acid and acid equivalents, whereinthe combined discharge slurry has an acidic pH and contains less freesulfuric acid and acid equivalents than the acid discharge slurry andmore free sulfuric acid and acid equivalents than the alkaline dischargeslurry, and wherein the valuable metal is gold and further comprising:after step (c) and before step (d), further increasing a pH of thecombined discharge slurry to a fourth pH, the fourth pH being equal toor higher than the third pH.
 14. The process of claim 8, wherein theacid discharge slurry comprises from about 2 to about 150 g/L freesulfuric acid and acid equivalents, wherein the alkaline dischargeslurry comprises no more than about 1 g/L free sulfuric acid and acidequivalents, wherein the combined discharge slurry has an alkaline pHand contains less acid consumers than the alkaline discharge slurry andmore acid consumers than the acid discharge slurry, and wherein thevaluable metal is gold and further comprising: after step (c) and beforestep (d), further increasing a pH of the combined discharge slurry to afourth pH, the fourth pH being higher than the first, second, and thirdpH's.
 15. A process, comprising: (a) pressure oxidizing, at a pH of lessthan pH 6.5 and in an acid autoclave, an acid generating andgold-containing feed material comprising an acid generator and acidconsumer to form an acid discharge slurry comprising generated acid andhaving a first pH of less than pH 6.5; (b) pressure oxidizing, at a pHof at least pH 6.5 and in an alkaline autoclave, an acid consuming andgold-containing feed material comprising the acid generator and acidconsumer to form an alkaline discharge slurry comprising sulfate sulfurand having a second pH of at least pH 6.5; (c) combining the acid andalkaline discharge slurries to provide a combined discharge slurryhaving a third pH greater than the first pH and less than the second pH;(d) further increasing a pH of the combined discharge slurry to a fourthpH, the fourth pH being higher than the first, second, and third pH's;and (e) thereafter recovering the gold from a solid phase of the pHadjusted combined discharge slurry.
 16. The process of claim 15, whereinthe acid generating and gold-containing feed material comprises moretotal moles of acid generators than acid consumers, and wherein the acidconsuming and gold-containing feed material comprises more total molesof acid consumers than acid generators.
 17. The process of claim 16,wherein an equivalent carbonate level in the acid generating andgold-containing feed material ranges from about 0.1 to about 1% byweight and wherein an equivalent carbonate level in the acid consumingand gold-containing feed material is at least about 0.7% by weight. 18.The process of claim 15, wherein the acid generator is primarily sulfidesulfur and wherein the acid discharge slurry comprises at least about 2g/L, but less than about 25 g/L, free sulfuric acid while the alkalinedischarge slurry comprises no more than about 1 g/L free sulfuric acid.19. The process of claim 18, wherein at least most of the sulfide sulfurin the acid generating and acid consuming feed materials is oxidized insteps (a) and (b) and wherein at least most of the oxidized sulfidesulfur in the acid and alkaline discharge slurries is in the form ofsulfate sulfur.
 20. The process of claim 15, wherein the acid dischargeslurry comprises from about 2 to about 150 g/L free sulfuric acid andacid equivalents, wherein the alkaline discharge slurry comprises nomore than about 1 g/L free sulfuric acid and acid equivalents, andwherein the combined discharge slurry has an acidic pH and contains lessfree sulfuric acid and acid equivalents than the acid discharge slurryand more free sulfuric acid and acid equivalents than the alkalinedischarge slurry.
 21. The process of claim 15, wherein the aciddischarge slurry comprises from about 2 to about 150 g/L free sulfuricacid and acid equivalents, wherein the alkaline discharge slurrycomprises no more than about 1 g/L free sulfuric acid and acidequivalents, and wherein the combined discharge slurry has an alkalinepH and contains less acid consumers than the alkaline discharge slurryand more acid consumers than the acid discharge slurry.